5,363 research outputs found
Changes in Cascading Failure Risk with Generator Dispatch Method and System Load Level
Industry reliability rules increasingly require utilities to study and
mitigate cascading failure risk in their system. Motivated by this, this paper
describes how cascading failure risk, in terms of expected blackout size,
varies with power system load level and pre-contingency dispatch. We used Monte
Carlo sampling of random branch outages to generate contingencies, and a model
of cascading failure to estimate blackout sizes. The risk associated with
different blackout sizes was separately estimated in order to separate small,
medium, and large blackout risk. Results from secure models of the IEEE
RTS case and a 2383 bus case indicate that blackout risk does not always
increase with load level monotonically, particularly for large blackout risk.
The results also show that risk is highly dependent on the method used for
generator dispatch. Minimum cost methods of dispatch can result in larger long
distance power transfers, which can increase cascading failure risk.Comment: Submitted to Transmission and Distribution Conference and Exposition
(T&D), 2014 IEEE PE
Cascading Power Outages Propagate Locally in an Influence Graph that is not the Actual Grid Topology
In a cascading power transmission outage, component outages propagate
non-locally, after one component outages, the next failure may be very distant,
both topologically and geographically. As a result, simple models of
topological contagion do not accurately represent the propagation of cascades
in power systems. However, cascading power outages do follow patterns, some of
which are useful in understanding and reducing blackout risk. This paper
describes a method by which the data from many cascading failure simulations
can be transformed into a graph-based model of influences that provides
actionable information about the many ways that cascades propagate in a
particular system. The resulting "influence graph" model is Markovian, in that
component outage probabilities depend only on the outages that occurred in the
prior generation. To validate the model we compare the distribution of cascade
sizes resulting from contingencies in a branch test case to
cascade sizes in the influence graph. The two distributions are remarkably
similar. In addition, we derive an equation with which one can quickly identify
modifications to the proposed system that will substantially reduce cascade
propagation. With this equation one can quickly identify critical components
that can be improved to substantially reduce the risk of large cascading
blackouts.Comment: Accepted for publication at the IEEE Transactions on Power System
Spectroscopy at the solar limb: II. Are spicules heated to coronal temperatures ?
Spicules of the so-called type II were suggested to be relevant for coronal
heating because of their ubiquity on the solar surface and their eventual
extension into the corona. We investigate whether solar spicules are heated to
transition-region or coronal temperatures and reach coronal heights (>6 Mm)
using multi-wavelength observations of limb spicules in different chromospheric
spectral lines (Ca II H, Hepsilon, Halpha, Ca II IR at 854.2 nm, He I at 1083
nm). We determine the line width of individual spicules and throughout the
field of view and estimate the maximal height that different types of off-limb
features reach. We derive estimates of the kinetic temperature and the
non-thermal velocity from the line width of spectral lines from different
chemical elements. We find that most regular spicules reach a maximal height of
about 6 Mm above the solar limb. The majority of features found at larger
heights are irregularly shaped with a significantly larger lateral extension
than spicules. Both individual and average line profiles in all spectral lines
show a decrease in their line width with height above the limb with very few
exceptions. Both the kinetic temperature and the non-thermal velocity decrease
with height above the limb. We find no indications that the spicules in our
data reach coronal heights or transition-region or coronal temperatures.Comment: Accepted for publication in Solar Physics, 52 pages, 32 figure
The polarization signature of photospheric magnetic fields in 3D MHD simulations and observations at disk center
Before using 3D MHD simulations of the solar photosphere in the determination
of elemental abundances, one has to ensure that the correct amount of magnetic
flux is present in the simulations. The presence of magnetic flux modifies the
thermal structure of the solar photosphere, which affects abundance
determinations and the solar spectral irradiance. We compare the polarization
signals in disk-center observations of the solar photosphere in quiet-Sun
regions with those in Stokes spectra computed on the basis of 3D MHD
simulations having average magnetic flux densities of about 20, 56, 112 and 224
G. This approach allows us to find the simulation run that best matches the
observations. The observations were taken with the Hinode SP, TIP, POLIS and
the GFPI, respectively. We determine characteristic quantities of full Stokes
profiles in a few photospheric spectral lines in the visible (630 nm) and
near-infrared (1083 and 1565 nm). We find that the appearance of abnormal
granulation in intensity maps of degraded simulations can be traced back to an
initially regular granulation pattern with numerous bright points in the
intergranular lanes before the spatial degradation. The linear polarization
signals in the simulations are almost exclusively related to canopies of strong
magnetic flux concentrations and not to transient events of magnetic flux
emergence. We find that the average vertical magnetic flux density in the
simulation should be less than 50 G to reproduce the observed polarization
signals in the quiet Sun internetwork. A value of about 35 G gives the best
match across the SP, TIP, POLIS and GFPI observations.Comment: 12 pages, 11 figures; accepted for publication in Ap
Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations
Numerical 3D radiative (M)HD simulations of solar convection are used to
understand the physical properties of the solar photosphere. To validate this
approach, it is important to check that no excessive thermodynamic fluctuations
arise as a consequence of the partially incomplete treatment of radiative
transfer. We investigate the realism of 3D convection simulations carried out
with the Stagger code. We compared the characteristic properties of several
spectral lines in solar disc centre observations with spectra synthesized from
the simulations. We degraded the synthetic spectra to the spatial resolution of
the observations using the continuum intensity distribution. We estimated the
necessary spectral degradation by comparing atlas spectra with averaged
observed spectra. In addition to deriving a set of line parameters directly, we
used the SIR code to invert the spectra. Most of the line parameters from the
observational data are matched well by the degraded simulation spectra. The
inversions predict a macroturbulent velocity below 10 m/s for the simulation at
full spatial resolution, whereas they yield ~< 1000 m/s at a spatial resolution
of 0.3". The temperature fluctuations in the inversion of the degraded
simulation do not exceed those from the observational data (of the order of
100-200 K rms for -2<log tau<-0.5). The comparison of line parameters in
spatially averaged profiles with the averaged values of line parameters in
spatially resolved profiles indicates a significant change of (average) line
properties at a spatial scale between 0.13" and 0.3". Up to a spatial
resolution of 0.3", we find no indications of the presence of excessive
thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that
simulations without spatial degradation contain fully realistic thermodynamic
fluctuations requires observations at even better spatial resolution.Comment: 21 pages, 15 figures + 2 pages Appendix, accepted for publication in
A&A; v2 version: corrected for an error in the calculation of stray-light
estimates, for details see the Corrigendum to A&A, 2013, 557, 109 (DOI:
10.1051/0004-6361/201321596). Corrected text and numbers are in bold font.
Apart from the stray-light estimates, nothing in the rest of the paper was
affected by the erro
- …